In fact, however, the birthrate for unmarried mothers, which had been steadily increasing since the early 1980s, peaked in 2008 and has declined 14 percent since, more than the decline for all women. The recent declines were sharpest among teenagers; black and Hispanic women; and those without a college degree — all of whom have typically had the highest rates of single motherhood — according to data from the Census Bureau and the National Center for Health Statistics.

There was only one group of unmarried women for whom the birthrate increased in recent years: those 35 and older. In many cases, they are having babies outside of marriage by choice, with more resources and education than the typical single mother.

They are still a small minority. But if these trends continue, single motherhood could become less of a sign of family instability. It could increasingly become one of the new ways people are choosing to form families, in an era when both marriage and divorce are declining.

“I don’t think people realize that there are a lot of older women now who are having babies deliberately, single mothers by choice,” said Isabel Sawhill, a senior fellow at the Brookings Institution and author of “Generation Unbound: Drifting Into Sex and Parenthood Without Marriage.”

In many cases, these women carefully planned to have children without a partner, she said. In others, they are living with a partner but not married, a pattern that is common in parts of Europe. Fifty-eight percent of out-of-wedlock births in the United States are to couples that live together, up from 41 percent in 2002, according to the government data.

Despite recent declines, single motherhood is still quite common: 40 percent of births in the United States are to unmarried women, and they are still more likely to be young, black or Latina and without a college degree. Some researchers who study the issue say that the recent slowdown might indicate that nonmarital births have reached their long-term level.

Some researchers and marriage advocates say the prevalence of single-parent families could have long-term negative effects. These families are more likely than two-parent ones to live in poverty. And children in low-income, single-parent households achieve lower levels of education and income over all, particularly boys.

The birthrate generally declines during recessions, and a large part of the recent decline in single motherhood is that the number of babies born over all has fallen 9 percent since 2007, as people have chosen to delay childbearing until they are more economically stable.

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Jennifer Williams, 43, at home in Pleasant Hill, Calif., with Maya, the daughter she conceived through sperm donation.CreditJason Henry for The New York Times

But that does not entirely explain the decline in out-of-wedlock births. As the economy has recovered, births among married women have increased again, but not among unmarried women. And some declines, as with teen pregnancy, started well before the recession.

June Carbone, a law professor at the University of Minnesota, sees the overall dip partly as a continuation of the trend of a decline in marriage because of the diminishing economic prospects for men. At first, women decided they didn’t need to marry to have a child, and now they might be deciding not to have a child at all.

“In the ’90s, I think the anti-abortion sentiment coincided with giving up on men — you’re not going to meet a guy worth marrying, you don’t have to have a shotgun marriage,” said Ms. Carbone, who co-wrote “Marriage Markets: How Inequality is Remaking the American Family.” She said, “My hunch is that the other shoe has dropped and people are just not having the kids.”

That pattern is most common among less educated women. During the recession, the decline in single motherhood was entirely attributable to women without college degrees, according to census data analyzed by Philip Cohen, a sociologist at University of Maryland who writes a blog called Family Inequality.

These are “women for whom the hardships of single motherhood are most acute,” Mr. Cohen said. “This could be deliberate planning, or it could reflect relationship problems or economic stress undermining their family plans.”

Among older women who are unmarried, ages 35 to 39, however, the birthrate was 48 percent higher in 2012 than in 2002, according to the National Center for Health Statistics. The increase was driven by college-educated women, according to Mr. Cohen’s analysis. “The delay in general fits a long-term pattern: that family formation is increasingly delayed until women are more established, spend more time in education and more time developing their careers,” he said.

Over all, older, highly educated women are more likely to have children than they were two decades ago, according to a Pew Research Center report published Thursday.

Many women who chose to have babies on their own after 35 used sperm donors. In interviews, some said they had not yet found a partner before the age when fertility plummets. “I was 40 and dating and dating and dating and just not having any luck,” said Jennifer Williams, now 43, a gerontologist in Pleasant Hill, Calif. She found a sperm donor and went through six rounds of attempts to get pregnant.

Her daughter, Maya, is four months old. “It’s absolutely the best thing I ever did,” she said.

Lizzie Skurnick, 41, a publisher of young adult fiction and a writer, wanted to become a mother, but didn’t want a partner at the time. She used donor sperm to conceive her son Javier, 1, and is considering having another baby in the same way. “If I had married any of the men I had dated, and they are lovely men, I would be carrying them also, because they always made less than I did,” she said. “Honestly, that’s just an additional stress on a household.”

Older women who chose to become single mothers said their decisions brought challenges. Some worried about the financial stress of raising a child on one income. Others feared that their children would miss having a father or a bigger family. Dating is expensive because of the added cost of a babysitter. “The most challenging thing is not having company at the end of the day after she goes to bed,” Ms. Williams said.

For Ms. Skurnick, “the one problem is it’s freaking hard on your back.”

But the benefits, they said, far outweighed the challenges. They avoided spousal arguments over child rearing, which they had seen tear apart friends’ marriages. They had autonomy in making parenting decisions. And by being older, they said, they had more stability. “I feel like I’ve had more life experience to be able to see we’re going to get through rough patches,” Ms. Williams said. Her career was established enough that she could start consulting after she decided she wanted a longer maternity leave than her job provided.

Many policy makers and researchers have shifted their focus from reducing the number of single mothers to bringing down the number of unintended pregnancies, which are riskier for both mothers and babies. Fifty-seven percent of out-of-wedlock pregnancies are unplanned — unchanged from 2002.

A version of this article appears in print on , on Page A12 of the New York edition with the headline: Single Motherhood, in Decline Over All, Rises for Women 35 and Older. Order Reprints | Today’s Paper | Subscribe